This application pertains to generation of radiation, especially extreme ultraviolet (EUV) radiation, from a laser-produced plasma (LPP), for the purpose of illuminating a reflective photomask in a lithography or mask inspection process. The application relates specifically to spectral purity filtering (eliminating unwanted out-of-band radiation wavelengths in the illumination) and power recycling (returning some of the out-of-band radiation to the plasma to enhance generation of usable, in-band radiation).
Current-generation EUV lithography systems use an LPP illumination source, which generates EUV radiation from laser-irradiated tin droplets. In this process a high-power, CO2 laser pulse (at a 10.6-μm wavelength) heats a small, molten tin target to form an ionized plasma, which generates EUV radiation from decay of tin ions to their neutral state. The optimum target size is much smaller than the diffraction-limited laser beam, so the target is typically first vaporized by a shorter-wavelength (e.g., 1-μm) pre-pulse laser to expand its size before it is irradiated and ionized by the main CO2 laser pulse. [Hori et al.]
LPP sources are also useful for EUV inspection and metrology, which do not need as much power as lithography, but which require a very small, high-brightness plasma source. For these applications, a relatively short-wavelength laser (e.g., 1-μm) can be used to ionize the target without pre-pulse irradiation. [Rollinger et al.]
FIG. 1 illustrates the primary components of a prior-art EUV lithography system. [Hori et al.; Migura et al.] The LPP source 101 comprises apparatus for generating the ionized plasma 102 (including the drive laser, pre-pulse laser, and tin droplet generator—not shown), and a collection mirror 103. The collection mirror focuses plasma-generated EUV radiation to an intermediate focus (IF) 104, where it is spatially filtered by an intermediate-focus aperture (IF aperture) 105. The aperture-transmitted radiation is conveyed by illumination optics 106 to a reflective photomask 107 at object plane 108. The illumination optics control characteristics of the illumination such as its spatial profile on the photomask, the illumination's numerical aperture, and the coherence factor. The EUV-illuminated photomask is imaged by projection optics 109, at reduced magnification, onto a semiconductor wafer 110 in image plane 111. The illumination optics typically expand the illumination to a ring field on the photomask, and the photomask and wafer are mechanically scanned in tandem to effect full-field exposure.
An EUV inspection or metrology system could be similar to the lithography system of FIG. 1, but it would not need a pre-pulse laser and the wafer would be replaced by an image sensor.
The collection mirror uses a multilayer reflective coating, typically comprising about 40 or more Mo/Si bilayers of approximate thickness 7 nm per bilayer, to reflect plasma-generated radiation. The collection mirror reflects useful “in-band” EUV within a 2% wavelength band centered at 13.5 nm. (The band is limited by the multiple EUV reflections between the plasma and the image plane.) But the mirror also reflects a large amount of plasma-generated “out-of-band” radiation from the deep ultraviolet (DUV) to long-wave infrared (IR), which can be detrimental to lithography processes. [Park et al.] A variety of prior-art techniques have been developed or proposed for reducing the undesired out-of-band radiation in the LPP source output.
Current-generation LPP sources reject the IR via diffractive scattering from a surface-relief grating on the collection mirror, as illustrated in FIGS. 2 and 3. [van den Boogaard et al. (2012); Medvedev et al. (2013); Trost et al.; Kriese et al.; Feigl et al.] A CO2 laser 201 irradiates the plasma 102 with IR radiation 202 (10.6-μm wavelength), and the plasma emits in-band EUV radiation 203 and out-of-band radiation including the 10.6-μm laser wavelength 204. A lamellar (rectangular-profile) diffraction grating on the collection mirror 103 separates the IR 205 from the EUV 206 in the reflected radiation.
An enlarged view of the mirror surface, illustrating the grating 301, is shown in FIG. 3. The grating comprises annular grooves, shown in cross-section. (The grating is axially symmetric around an optical axis through the plasma 102 and IF 104.) The grating is configured to extinguish zero-order (i.e., undiffracted) IR radiation at the 10.6-μm drive-laser wavelength, scattering the reflected IR into first (±1) and higher diffraction orders.
FIGS. 2 and 3 illustrate a light cone 207 converging from the plasma to a particular mirror point 208. The grating structure near this point diffracts 10.6-μm IR into ±1-order diffracted beams with light cones 209 and 210. The grating period Λ is too long to significantly affect the EUV (13.5-nm) radiation, which is substantially undeviated from the zero order beam, indicated as light cone 211. The collection mirror has an ellipsoidal substrate shape with foci at the plasma 102 and the IF 104 so that zero-order reflected EUV radiation is focused toward the IF and through the IF aperture 105. The IR is diffractively scattered out of the IF aperture.
For near-normal incidence the zero-order IR is extinguished by making the grating height h (FIG. 3) approximately one-quarter of the wavelength (i.e., 2.65 μm to achieve zero-order extinction of the 10.6-μm laser wavelength). The angular deviation θ between the zero and first diffraction orders is roughly equal to the wavelength-to-period ratio (at the laser wavelength); e.g., for a typical grating period of 1 mm the IR laser wavelength is diffractively deviated by approximately (10.6 μm)/(1 mm), or 10 mrad. By comparison, the plasma source's subtend angle δ at the grating is typically of order 1 mrad (e.g., for a 200-μm plasma diameter and a 200-mm collection mirror focal length). All of the light cones 207, 209, 210, and 211 have roughly 1 mrad extent, so the 10-mrad IR scatter angle θ is more than sufficient to separate the first-order IR and zero-order EUV beams.
The grating also induces some diffractive scatter in the EUV, but the scatter angle is only of order (13.5 nm)/(1 mm), i.e., 13.5 μrad, which is insignificant in comparison to the plasma's 1-mrad angular extent.
A limitation of these types of systems is that they are generally designed to only extinguish the zero order at only one wavelength (10.6 μm), so they do not achieve full rejection of all out-of-band radiation. Feigl et al. describe a two-level grating structure that rejects two wavelengths (the drive laser's 10.6-μm wavelength and the pre-pulse laser's 1.06-μm wavelength). But it does not fully exclude other wavelengths, including the DUV spectrum.
The grating 301 typically has the form illustrated in FIG. 4A. A lamellar, surface-relief structure is patterned in a substrate 401, and a multilayer reflective film 402 is then deposited on the grating structure. But van den Boogaard et al. (2012) use a different approach, as illustrated in FIG. 4B: The multilayer reflective coating is deposited on a smooth substrate, which does not have a grating topography, and the lamellar grating structure is patterned directly in the multilayer film.
Moriya et al. (U.S. Pat. No. 8,592,787) similarly disclose a spectral-filter grating structure patterned in a multilayer film on a smooth substrate, but the structure is non-lamellar. For example, the illustrated “Embodiment 1” grating in FIG. 3 of Moriya et al., shown as FIG. 4C herein, comprises a blazed, sawtooth profile, which diffracts the drive-laser (10.6-μm) radiation out of the IF aperture. The grating operates functionally as illustrated in FIG. 2, although its structure differs from the lamellar grating illustrated in FIG. 3. The reflected in-band EUV is concentrated in or near the zero order, which intercepts the IF aperture, and the out-of-band radiation is diffractively diverted out of the IF aperture. (See Moriya et al. at 13:16-14:6 and 14:57-63.)
A drawback of the Moriya et al. design is that it requires many layers in the reflective film. For example, the exemplary “Embodiment 1” has 300 bilayers including an unpatterned, 50-bilayer base structure and a patterned, 250-bilayer grating structure. (See Moriya et al. at 11:44-61 and 13:4-10) A conventional multilayer reflective film can achieve high EUV efficiency with only approximately 50 bilayers, but Moriya et al. note (at 12:8-12) that “If the number of pair layers is less than 100, then . . . it is not possible sufficiently to separate the EUV radiation from the radiation of other wavelengths.”
The Embodiment 1 structure of Moriya et al. is specified as having 250 patterned bilayers with a bilayer thickness of 6.9 nm, implying a grating height of 1.7 μm. This is only about 16% of the 10.6-μm laser wavelength, but the height would actually need to be approximately 5.3 μm (one-half wavelength) to achieve first-order blazing and zero-order extinction at the laser wavelength. This would require approximately 768 patterned bilayers. With 250 patterned bilayers only a minor portion of the 10.6-μm IR would be diverted out of the IF aperture.
Moriya et al. cite prior-art proposals for spectral filters that use a blazed diffraction grating to separate the EUV from out-of-band radiation by diffracting the in-band EUV, rather than IR. [Chapman (U.S. Pat. No. 7,050,237); Bristol (U.S. Pat. No. 6,809,327); Kierey et al.; Sweatt et al. (U.S. Pat. No. 6,469,827)]
Chapman discloses an EUV-diffracting grating formed by cutting a thick, multilayer EUV-reflection coating at an inclined angle. A disadvantage of this type of grating is that it requires a very large number of Mo/Si bilayers (“at least two thousand” as recited in Chapman's claim 1).
Bristol and Kierey et al. disclose an EUV-diffracting grating disposed in a converging beam to separate the EUV from out-of-band radiation on a focal plane. Two disadvantages of this type of system are that it requires a separate optical element for spectral filtering, and the additional element significantly reduces EUV throughput.
Sweatt et al. disclose an alternative spectral filtering method that also uses a blazed diffraction grating to diffract the EUV and separate the EUV from out-of-band radiation. In some embodiments the grating is a near-normal-incidence reflective element, but the grating fabrication process differs from that of Moriya et al. Sweatt et al. note that “the blazed grating is preferably constructed on a substrate before a reflective multilayer, e.g., alternating Si and Mo layers, is deposited over the grating.” Gratings of this type are described in Voronov et al. FIG. 5 illustrates the grating structure in cross section. A surface-relief structure having a blazed, sawtooth profile is patterned in a substrate 501, and a multilayer reflective film 502 is then deposited on the structure.
The condenser mirror disclosed by Sweatt et al. does not focus the EUV radiation through an intermediate focus and illumination optics as in FIG. 1. Instead, it focuses the plasma source onto a ring image, which is projected directly onto the photomask. The system has limited practical utility because it lacks the illumination control capabilities of the illumination optics 106 in FIG. 1. Also, the system uses a filtering aperture in close proximity to the photomask (element 124 in FIG. 8 of Sweatt et al.), which could create problems with heat dissipation, optical back-scatter of out-of-band radiation, and mechanical clearance (e.g., interference with a photomask pellicle and wafer loading mechanics). These limitations do not exist with the prior art represented in FIG. 1. Chapman describes other limitations of the Sweatt et al. system, as understood in the prior art. (See Chapman at 2:25-43.)
Blazed EUV reflection gratings operating at near-normal incidence have been researched by Liddle et al. and by van den Boogaard et al. (2009), although it is unclear from these publications how such gratings might be incorporated into an LPP collector for spectral filtering.
Other spectral filtering methods that do not use grating diffraction have also been proposed. Chkhalo et al. and Suzuki et al. disclose free-standing transmission films that transmit EUV and reflect IR, but the fragility of the film and its EUV transmission loss make such films impractical. The collection mirror's multilayer reflective film can be designed to reflect EUV and suppress IR. [Medvedev et al. (2012)] This avoids the need for a separate, fragile transmission film, but the EUV reflection efficiency is significantly compromised.
In most prior-art spectral filtering systems the rejected out-of-band radiation is eliminated as waste heat. But Bayraktar et al. disclose an IR-diffracting grating that is similar to FIG. 2, except that one of the first diffraction orders at 10.6 μm is directed back onto the plasma to enhance generation of in-band EUV radiation by the plasma. This “power recycling” capability could help boost EUV in-band power at intermediate focus to 250 W, the industry target level at which EUV lithography can become commercially viable for high-volume semiconductor manufacture. (Current state-of-the-art LPP sources achieve about 100 W.) But the Bayraktar et al. power recycling method has several practical limitations: It is only able to recycle out-of-band radiation at one wavelength (the 10.6-μm drive-laser wavelength); it can only recycle radiation that intercepts the collection mirror; and the grating's diffraction efficiency at 10.6 μm is only about 37%.